High tension circuit breakers fitted with varistors are known, particularly for switching shunt reactances in electricity grids. The purpose of having varistors, also referred to as "non-linear resistors" or as "variable resistors" or as "voltage-dependent resistors", is to reduce surges.
The lower the operating threshold of a varistor, the more effective the protection it provides against surges. The higher the surge, the greater the energy that is absorbed by the varistor.
The desired surge limit is often of the order of 1.5 p.u. to 1.6 p.u.
For line circuit breakers fitted with varistors, there is a major problem that needs to be considered: firstly it is desirable to fix the surge threshold at 1.5 p.u., and secondly, in phase opposition, surge may reach 2 p.u. to 2.5 p.u.
Proposals have been made to connect an interrupter in series with the varistor in order to avoid applying too high a tension to the varistor. However, in spite of this disposition, the varistor remains the seat of excessive energy dissipation. Consider phase opposition by way of example. On circuit breaking: a tension of about 2 p.u. is applied to the varistors for one period (at 50 Hz or at 60 Hz) before the arc is finally extinguished on the contacts of the interrupter.
At a tension of 2 p.u., the current may reach a high value. For example:
at 1 p.u. the current may be 5/10,000 amps; PA0 at 1.5 p.u. the current may be 1 amp; and PA0 at 2 p.u. the current may exceed 2,000 amps.
Since the voltage in the range 1.5 p.u. to 2 p.u. may be applied for a duration of several milliseconds, the energy dissipated in the varistors may reach several thousand kilojoules.
It is necessary to reduce this energy while still ensuring proper operation at 1.5 p.u.
The varistors may also be subjected to thermal overload in the event of tension being conveyed to the terminals of a multiple chamber circuit breaker by partial arcing when circuit breaking on a fault or on an empty line.
An object of the present invention is to provide a high tension circuit breaker having varistors and providing a solution to this problem.